1. Field of the Invention
The present invention relates to an apparatus and a method for controlling an actuator in an optical image stabilizer to prevent an image blur from occurring independently of a hand-shake of a low frequency and a hand-shake of a high frequency by optimally controlling the actuator in the optical image stabilizer for both of the hand-shake frequencies.
2. Description of the Related Art
An optical image stabilizer (OIS) senses a user's hand-shake using a motion sensor, for example, a gyro sensor, and prevents an image blur by moving a lens module or an image sensor in an opposite direction to a hand-shake direction by controlling an actuator.
A voice coil motor (VCM) is a frequently used actuator in the OIS for moving the lens module or the image sensor. The VCM has a coil mounted outside a camera lens module to control a direction and an amount of current flowing in the coil and to move the lens module to a desired position.
The major factor to cause the image blur relies on how much light is transferred to RGB pixels of the image sensor. As the camera lens module is shaken, light is transferred to a wider range of pixels, causing the image blur. The OIS moves the position of the lens module or the image sensor to prevent or reduce the image blur.
To prevent the image blur from occurring by moving the lens module, a proportional-integral-derivative (PID) controller is used to control the actuator or VCM. Considering characteristics of the PID controller, it is difficult to accurately control both the low and high hand-shake frequencies.
When the hand-shake of the high frequency is stabilized, a motion value of the hand-shake of the low frequency is excessively over calculated. Because of this, although the lens module does not actually need to move much, the lens module may excessively move. When the PID controller is designed to cope with any one of the low and high hand-shake frequencies, in the case in which the hand-shake frequency is suddenly changed or is in the frequency environment in an opposite direction thereto, excessive occurrences of image blur may be difficult to control.
The performance of the PID controller is changed based on the optimization of ratios of three coefficients, of a proportional coefficient Kp, an integral coefficient Ki, and a differential coefficient Kd, in the given environment. The proportional coefficient Kp has a connection with a current control value, the integral coefficient Ki has a connection with a past control value, and the differential coefficient Kd has a connection with a future control value.
The proportional coefficient Kp has a connection with how large a weight puts on a current error, the integral coefficient Ki has a connection with how many past errors based on the present are integrally controlled, and the differential coefficient Kd has a connection with how to control errors based on a value to some degree and a direction in the future.
One of the values of the three PID coefficients has a close effect on the remaining two coefficient values, and therefore even in the configuration of the PID controller, other PID coefficients may be determined to be optimal values depending on the environment.
Only the three coefficients may be changed in real time by the PID controller. Since the three coefficients may be optimized in the given environment and have various values depending on the purpose, it is difficult to meet all the conditions using one setting value in the OIS. Thus, a trade-off occurs in the performance of the PID controller based on where a frequency domain to be intensively controlled is found.
At the time of driving the OIS, assume the case in which a hand is quickly shaken or slowly shaken. The PID controller sets the Kp, the Ki, and the Kd to prevent a hand from quickly shaking. In this case, when the same Kp, Ki, and Kd are used to prevent the hand-shake of the low frequency, the output from the PID controller is excessively changed in the low frequency domain, such that an oscillation signal may be output from the VCM.
When the excessive oscillation signal is generated from the VCM, the camera lens module oscillates, and thus a captured image is excessively shaken, which is not a preferable phenomenon. To prevent this, since the Kp, the Ki, and the Kd are set to tune a low frequency to correct the hand-shake of the low frequency, when the hand-shake of the high frequency occurs, the hand-shake itself may not be compensated. The reason is that a change in current values moving the lens module is small in the VCM and thus the lens module may not be located immediately at a desired place after the hand-shake phenomenon by slowly moving the lens module.
For example, Japanese reference, JP2010-061160, describes a biasing correction apparatus and a photographing apparatus, which prevent a biasing correcting lens from unstably moving around a moving center at the time of triangular fixing with a small biasing amount by strongly applying a velocity bias using a linear equation velocity bias table when a triangular fixing state is determined by a support state determination unit. In JP2010-061160, it is difficult to optimally control the VCM of the OIS for both the low and high hand-shake frequencies.